In recent years, a semiconductor laser excitation solid laser using a semiconductor laser as an excitation light source of a solid laser is attracting people's attention. In particular, a semiconductor laser excitation solid laser emitting a laser light of green color (wavelength: 532 nm) is expected as a replacement light source of a semiconductor laser because a semiconductor laser emitting a laser light of green color is not realized yet.
A semiconductor laser excitation solid laser emitting a laser light of green color is used for various purposes such as a measurement apparatus, a guide light of an infrared laser, marking for positioning, and the like by utilizing the property that the spectral luminous efficiency of green color is extremely high. Also, in recent years, it is expected to be used as a green color light source of various image formation devices such as a color projector, a color projection television set and the like.
In general, not only the above-described semiconductor laser excitation solid lasers emitting a laser light of green color but also the semiconductor laser excitation solid lasers emitting a laser light having various wavelengths desirably have a stable light quantity of the output light thereof irrespective of the purposes for which they are used. In particular, when a semiconductor laser excitation solid laser is used as a light source of an image formation device as be described above, it is strongly desired that the light quantity of the output light is stabilized because the light quantity of the output light directly affects the color reproducibility or the like of the image formation device.
For this reason, Japanese Patent Application Laid-Open (JP-A) No. 10-93166, for example, proposes a technique of stabilizing a light quantity of an output light by branching the laser light emitted from a semiconductor laser excitation solid laser into the output light and a control light with use of a beam splitter, and controlling the light quantity of the laser light emitted from the semiconductor laser excitation solid laser (controlling the driving current of the semiconductor laser) in accordance with the light quantity of the control light detected by a light reception element (so that the detected light quantity will be constant).
However, even if the technique disclosed in JP-A No. 10-93166 is applied, a phenomenon occurs such that the light quantity of the output light of the semiconductor laser excitation solid laser is not sufficiently stabilized when it is accompanied by an environmental change such as a temperature change. For this reason, as a realistic means for sufficiently stabilizing the light quantity of the output light, there is a need to maintain the temperature of the semiconductor laser excitation solid laser to be constant with use of a suitable temperature controller. This accompanies an increase in the scale of the apparatus as a whole or a rise of the costs, thereby raising a problem of being unpracticable.